Background
Alley et al. (2002, 2003) contend that "human forcing of climate change" - by which they mean CO2-induced global warming - "is increasing the probability of large, abrupt events."  Speaking of temperature increases of "as much as 10°C change in 10 years," they state that these changes "can occur when gradual causes push the earth system across a threshold."  In this regard, they further state that "warming and the associated changes in the hydrological cycle constitute a threshold for the THC," or thermohaline circulation of the world's oceans, and that "once reduced, the THC is more susceptible to perturbations," additionally noting that "very close to a threshold, the evolution of the THC loses predictability altogether."  Simply put, Alley et al. claim that warming will vastly increase the instability of earth's climate.

What was done
Noting that still other studies, including those of Manabe and Stouffer (1993) and Rahmstorf and Ganopolski (1999), "have predicted that enhanced precipitation at higher latitudes in a warmer world could enhance thermohaline (and hence regional climate) instability," Draut et al. decided to test this hypothesis by evaluating climate variability throughout a portion of the mid-Pliocene, which they say "is an ideal interval in which to investigate prolonged warm conditions at high resolution," as this period "is the most recent in geologic time with temperatures comparable to those projected for the 21st century."  Hence, they studied deep-sea sediment cores retrieved from Ocean Drilling Program Sites 980 and 981 (55°29'N, 14°39'W) that span the mid-Pliocene from about 3.1 to 3.3 million years ago.

What was learned
Draut et al. say their findings suggest that "no strong temperature-related forcing mechanism affected thermohaline circulation during the mid-Pliocene warm period," specifically stating that this observation "contrasts with scenarios predicted for 21st century global warming by several modeling studies, in which warm conditions are associated with enhanced freshwater runoff and cessation of thermohaline circulation in the North Atlantic, resulting in long-term cooling."  Instead, they report that their data point "to a relatively stable climate during the mid-Pliocene warm period, analogous to millennial-scale stability within the Holocene and Marine Isotope Stage 11," noting further that "millennial-scale climate fluctuations appear to occur with reduced amplitude during warm episodes," additionally citing Oppo et al. (1998), McManus et al. (1999, 2003) and McIntyre et al. (2001) in support of this statement.

What it means
Clearly, there is a wealth of real-world data that indicates that increasing warmth leads to increasing climate stability, which is just the opposite of what many models - but not all [see, for example, Saenko et al. (2003)] - predict to occur in response to further global warming.  When it comes to choosing between contradictory model predictions and real-world observations, therefore, we'll take the world of reality every single time it has something to tell us.  And it's been telling us for quite a while now that warmth brings climate stability.

References
Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M.  2002.  Abrupt Climate Change: Inevitable Surprises. National Research Council, National Academy Press, Washington, DC.

Alley, R.B., Marotzke, J., Nordhaus, W.D., Overpeck, J.T., Peteet, D.M., Pielke Jr., R.A., Pierrehumbert, R.T., Rhines, P.B., Stocker, T.F., Talley, L.D. and Wallace, J.M.  2003.  Abrupt climate change.  Science 299: 2005-2010.

Manabe, S. and Stouffer, R.J.  1993.  Century-scale effects of increased atmospheric CO2 on the ocean-atmosphere system.  Nature 364: 215-218.

McIntyre, K., Delaney, M.L. and Ravelo, A.C.  2001.  Millennial-scale climate change and oceanic processes in the late Pliocene and early Pleistocene.  Paleoceanography 16: 535-543.

McManus, J.F., Oppo, D.W. and Cullen, J.L.  1999.  A 0.5-million-year record of millennial-scale climate variability in the North Atlantic.  Science 283: 971-974.

McManus, J.F., Oppo, D.W., Cullen, J.L. and Healey, S.L.  2003.  Marine isotope stage 11 (MIS 11): Analog for Holocene and future climate?  Geophysical Monograph Series 137: 69-85.

Oppo, D.W., McManus, J.F. and Cullen, J.L.  1998.  Abrupt climatic events 500,000 to 340,000 years ago: Evidence from subpolar North Atlantic sediments.  Science 79: 1335-1338.

Rahmstorf, S. and Ganopolski, A.  1999.  Long-term global warming scenarios computed with an efficient coupled climate model.  Climatic Change 43: 353-367.

Saenko, O.A., Weaver, A.J. and Schmittner, A.  2003.  Atlantic deep circulation controlled by freshening in the Southern Ocean.  Geophysical Research Letters 30: 10.1029/2003GL017681.